A conventional and well-known multi-cylinder internal combustion engine adopting an electronic control fuel injection apparatus comprises one fuel injector per one of intake ports, each communicating with each of the cylinders (or each of combustion chambers of the cylinders). Further, the internal combustion engine of this type comprises a three-way catalytic converter (catalyst) and an air-fuel ratio sensor in an exhaust passage. The electronic control fuel injection apparatus, by an air-fuel ratio feedback amount calculated based on an output of the air-fuel ratio sensor, controls an “amount of a fuel injected from each of the fuel injectors” in such a manner that an average of air-fuel ratios of mixtures supplied to the entire engine coincides with (becomes equal to) the stoichiometric air-fuel ratio.
Meanwhile, when a property (characteristic) of the fuel injector for a specific cylinder becomes a “property that the fuel injector injects fuel in an amount larger (more excessive) than an instructed fuel injection amount”, only an air-fuel ratio (air-fuel-ratio-of-the-specific-cylinder) of an air-fuel mixture supplied to the specific cylinder shifts to an extremely richer side. That is, a non-uniformity among air-fuel ratios of the cylinders (a variation in air-fuel ratios among the cylinders, air-fuel ratio imbalance among the cylinders) becomes large (prominent). In other words, there arises an imbalance among air-fuel ratios, each of which is an air-fuel ratio of a mixture supplied to each of a plurality of the cylinders (i.e., air-fuel ratios of individual cylinders).
In this case, an average of the air-fuel ratios of the mixtures supplied to the engine becomes an air-fuel ratio richer (smaller) than the stoichiometric air-fuel ratio. Accordingly, the air-fuel ratio of the specific cylinder is shifted to a leaner (larger) air-fuel ratio so that the air-fuel ratio of the specific cylinder is made closer to the stoichiometric air-fuel ratio, by the air-fuel ratio feedback amount calculated based on the output value of the air-fuel ratio sensor. The air-fuel ratio of the specific cylinder is still richer than the stoichiometric air-fuel ratio. Further, each of the air-fuel ratios of the other cylinders (air-fuel ratios of the cylinders other than the specific cylinder) is caused to shift to a leaner (larger) air-fuel ratio so that the air-fuel ratios of the other cylinders are caused to deviate more from the stoichiometric air-fuel ratio, by the air-fuel ratio feedback amount. At this time, since the number of the other cylinders is larger than the number (which is one) of the specific cylinder, each of the air-fuel ratios of the other cylinders is slightly leaner (larger) than the stoichiometric air-fuel ratio. As a result, the average of the air-fuel ratios of the entire mixtures supplied to the engine is caused to become roughly equal to the stoichiometric air-fuel ratio.
However, the air-fuel ratio of the specific cylinder is still richer (smaller) than the stoichiometric air-fuel ratio, and the air-fuel ratios of the other cylinders are still leaner (larger) than the stoichiometric air-fuel ratio, and therefore, a combustion condition of the mixture in each of the cylinders is different from a perfect combustion condition (combustion condition obtained when an air-fuel ratio of a mixture is equal to the stoichiometric air-fuel ratio). As a result, an amount of emissions (an amount of unburnt substances and/or an amount of nitrogen oxides) discharged from each of the cylinders increases. Accordingly, even though the average of the air-fuel ratios of the mixtures supplied to the engine coincides with the stoichiometric air-fuel ratio, the three-way catalytic converter may not be able to purify the increased emissions, and thus, there is a possibility that the emissions become worse. It is therefore important to detect whether or not the air-fuel ratio non-uniformity among cylinders becomes excessively large, since an appropriate measure can be taken in order to prevent the emissions from becoming worse.
One of conventional apparatuses (apparatuses for determining an air-fuel ratio imbalance among cylinders) that determine whether or not the “non-uniformity of the air-fuel ratios among cylinders (air-fuel ratio imbalance among cylinders, imbalance among air-fuel ratios of individual cylinders)” becomes excessively large obtains an estimated individual cylinder air-fuel ratio which represents each of the air-fuel ratios of the cylinders by analyzing an output of a single air-fuel ratio sensor disposed at an exhaust gas aggregated portion. Thereafter, the conventional apparatus determines whether or not the “non-uniformity of the air-fuel ratios among cylinders” becomes excessively large based on the estimated individual cylinder air-fuel ratio (refer to, for example, Japanese Patent Application Laid-Open (kokai) No. 2000-220489).